The invention relates generally to electrical machines and, more particularly, to a system and method for smoothing a salient rotor of an electrical machine in order to reduce friction and windage losses in the machine, while providing for a mechanically robust rotor assembly that can withstand centrifugal forces at high speeds.
The usage of electrical machines in various industries has continued to become more prevalent in numerous industrial, commercial, and transportation industries over time. Several types of such electrical machines, such as reluctance motors and stator permanent magnet motors for example, require the use of salient poles or protruding teeth on the rotor. These types of rotors are generally passive and robust and thus suitable for high-speed applications. It is recognized, however, that the salient structure of such rotors contributes to the creation of excessive windage losses due to the tendency of the protrusions to catch air as the rotor rotates.
One manner of addressing the issue of windage losses that accompany the use of salient rotors is to “smooth” the outer surface of the rotor by filling the interpolar spaces between the rotor teeth. One prior art mechanism for filling in the interpolar spaces is magnetic bridges that are positioned between the rotor poles so as to connect adjacent rotor pole tips, thereby smoothing the rotor. Such magnetic bridges, however, negatively affect the magnetic saliency and thus serve to reduce the performance of the electrical machine. Another prior art mechanism for filling in the interpolar spaces is segments of a non-magnetic material (e.g., stainless steel) positioned between the rotor poles, such as described in U.S. Pat. No. 4,916,346, that provide smoothing to the rotor and do not affect the magnetic saliency. However, in the prior art, such non-magnetic segments have been formed as solid, plain metallic inserts that greatly increase the overall mass of the electrical machine. Additionally, such non-magnetic segments have been either welded to the rotor poles or secured thereto solely via a notch formation, both of which may not be ideal for retaining the segments between the rotor poles and/or provide a desired robustness for the rotor assembly. Further, metallic inserts and their associated welds are subject to eddy current losses that reduce the efficiency of the machine.
Therefore, it would be desirable to provide inserts for smoothing a salient rotor that address the issue of windage losses without affecting the magnetic performance. It would further be desirable for such inserts to be assembled with the salient rotor in a mechanically robust way that can withstand centrifugal forces at high speeds, while minimizing the mass that is added to the rotor.